Unlike most in vitro systems developed to study HIV replication, humans infected with HIV are co- infected with a variety of pathogenic and nonpathogenic microbes. Coinfections may influence the clinical outcome of either infection. For example, HIV infection accelerates the course of hepatitis C virus (HCV) related hepatic disease while persistent infection with GB virus type C (GBV-C) is associated with prolonged survival in HIV-infected individuals in all studies conducted prior to HAART. The mechanism(s) by which microbial coinfections interact to alter disease course provide insight into HIV natural history, and may identify novel therapeutic targets or assist in the development of prevention strategies. GBV-C is a member of the family Flaviviridae and GBV-C is a common infection of humans. The virus replicates in human peripheral blood mononuclear cells (PBMCs) including B and T lymphocytes (CD4+ and CD8+ subsets). Co-infection of PBMCs with GBV-C and HIV results in inhibition of HIV replication, and this is mediated in part by downregulation of HIV coreceptors and by induction of soluble antiviral factors. GBV-C inhibits the replication of both CCR5- and CXCR4-tropic HIV isolates, and inhibits diverse HIV clades from around the world. In addition, GBV-C infection appears to block CD4 expansion in people receiving IL-2 therapy and is associated with decreased T cell activation as measured by surface expression of several markers (CD38, CCR5, CD25, CD69). Addition of the envelope glycoprotein E2 reproduces some of these effects (CCR5 regulation, HIV inhibition) although it has not been thoroughly studied. Since GBV-C E2 protein, when added to cells independent of GBV-C replication mediates HIV replication inhibition, it is likely that this is mediated by interactions between E2 and its cellular receptor or receptors. To examine this hypothesis, we plan to characterize interactions between GBV-C E2 and it's cellular receptor(s), identify and validate candidate E2 receptors, determine the domains on E2 that interact with cellular receptors, and characterize how GBV-C E2 modulates cellular cytokines, chemokines and receptors. Preliminary data have identified candidate receptors, and two of the five initial candidates are lymphocyte surface proteins that are important in both HIV replication and host immune responses. We propose four aims to further identify candidate receptors, validate that these receptors interact with E2, and we will assess the effect of E2 and GBV-C particle interaction with lymphocytes (both replicating and not) on chemokine receptor expression, T cell activation, and induction of anti-HIV chemokines. Finally, we will begin characterization of the domain within E2 that is responsible for binding and modulating T cell receptor expression. This information has the potential to facilitate development of cellular-based, anti-HIV and immunomodulatory treatments. PUBLIC HEALTH RELEVANCE: The VA healthcare system is the largest provider for HIV-infection people in the United States, and although HIV can now be managed by antiretroviral therapy (ART), ART is expensive, has significant side effects, and resistance develops easily, particularly in patients who are not strictly adherent to therapy. GB virus C is a common human virus that does not appear to cause any disease, and studies show that it grows in the same cells as HIV resulting in inhibition of HIV replication. This application proposes to study how one of the GBV-C virus proteins (E2) modifies T cells so that HIV does not grow as well, and will serve as the basis for developing new approaches to HIV therapy.